We made expeditions to a total of 12 countries for 540 days during 1995-1998 fiscal years and obtained the following results. 1. Adaptive significance : (1) Physiological characteristics : Immunohistochemical analysis showed two types of chloride cells in the gill lamella and gill fillament of Anguilla japonica. The former was activated in freshwater habitat and the latter, in the marine habitat following the downstream migration to the sea. (2) Migratory behavior : The survey on the inshore migration of tropical glass eels during 1995-1998 revealed that three species (A.celebesensis, A.marmorata and A.bicolor pacifica) occurred at the mouth of Poigar River, Sulawesi Island, Indonesia. The former two migrated almost throughout the year with the peak season in June and the last occurred in Jan., Mar., Apr., Oct. and Dec.. Analyses on otolith microstructure and Sr/Ca ratio suggested that tropical eels metaborphose at 90-160 days after hatching while teperate species, A.australis and A.di
… Moreeffenbachi collected in New Zealand, metamorphosed at older ages, 204 and 248 days, respectively. Age at recruitment to freshwater was also older in temperate eels (268 and 297 days in mean) than those in tropical eels (116-195 days). These differences in early life history of eels might be an adaptation which was caused on the evolutional process from local migaration in tropical eels to long distance migration in temperate species. 2. Evolition : (1) Phylogenetic analysis : Phylogenetic analysis on mtDNA data set suggested that Anguilla originated in tropical area in the Cretaceous and that the ancestor of A.anguilla and A.rostrata would dispersed from the Indo-Pacific into the Atlantic Ocean through the Tethys Sea at latest 35 Ma ago. (2) Parasitological analysis : Two new species of Pseudodactylogyrus, monogenean parasites, were found on the gills of the two Australian eels, A.australis and A.reinhardti. They appear to be endemic to Australia, distinct morphologi- cally and genetically from their nearest relatives infecting the Japanese eel, indicating that Pseudodactylogyrus has coevolved with eels during the isolation of A.japonica in eastern Asia from the two species in Australia.1.適応機構:(1)生理学的特性:Anguilla japonicaについて浸透圧調節機能に深く係わる塩類細胞を免疫組織化学的手法により検出した。淡水生活期には二次鰓弁上の塩類細胞が活性化するが,降河回遊に伴ってこれは退縮し,逆に海洋生活期に必須の一次鰓弁上の塩類細胞が活性化することがわかった。(2)回遊生態:スラウェシ島のポイガル川河口において熱帯ウナギの接岸・遡河生態を調べた所,A.celebesensis,A.marmorataおよびA.bicolor pacificaの3種が出現し,前二者の遡河盛期は6月で,ほぼ周年にわたり接岸することがわかった。またA.bicolor pacificaは量的に少なく,1,3,4,10,12月に出現することが明らかになった。一方,南半球の温帯種としてニュージーランドのA.australis schmidtiとA.dieffenbachiの耳石日周輪とSr/Ca比を調べ,スラウェシ島の熱帯3種と比較した。前者はそれぞれ平均204日と248日で変態を開始するに対し,熱帯種は90〜160日と若齢で変態し,熱帯ウナギに対して温帯ウナギの初期生活史は遅延されているものと考えられた。2.類縁系統関係;(1)分子系統関係:世界中のウナギ属魚類全18種・亜種を採集して,そのミトコンドリアDNA塩基配列データによる分子系統解析を行ったところ,ボルネオ島にのみ分布する熱帯ウナギA.borneensisが最も古い系統であり,ウナギ属魚類は白亜紀に熱帯で起源したことが示された。(2)寄生虫学的検討:オーストラリアの7地点で採集したA.australisとA.reinhardtiの寄生虫検査によって,鰓寄生の単生虫Pseudodactylogyrusの2新種を見出した。ウナギの祖先型に寄生していた単生虫がそれぞれの地域で宿主と共に進化したものと考えられた。 Less